A fuel cell has been proposed as a clean, efficient, and environmentally responsible energy source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative for the traditional internal-combustion engine used in modern vehicles. One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of supplying a quantity of electricity sufficient to provide power to a vehicle.
As is well understood in the art, the membranes within the fuel cell stack must have a certain relative humidity (RH) for efficient performance. Measures are often taken to maintain the membrane hydration within a desired range that optimizes proton conduction across the membranes. For example, in U.S. Pat. No. 6,376,111, hereby incorporated herein by reference in its entirety, a controller utilizes feedback to control the humidity of a fuel cell assembly. Humidifiers or water vapor transfer (WVT) devices are commonly used to humidify inlet reactant gases provided to the fuel cell stack. Thermal control strategies based on a relationship between RH and fuel cell temperature, as measured by the coolant temperature, for example, have also been employed in controlling membrane hydration. Other fuel cell parameters, such as stoichiometry and pressure, are further known to affect fuel cell humidification.
The level of humidification in fuel cell systems of the art has been controlled in response to a variety of feedback indicators including inlet RH, outlet RH, temperature, pressure, flow rate, and electrical current measurements. However, typical sensors employed in measuring these indicators often exhibit drift and may be unreliable. Relative humidity sensors, in particular, are known to have limited use in fuel cell applications due to corrosion and swelling of the sensors with repeated exposure to liquid water. Thus, typical sensors have not been desirably effective for purposes of humidification feedback-control in fuel cell systems.
High frequency resistance (HFR) has previously been used as an offline lab diagnostic technique for indirectly measuring MEA hydration in the fuel cell. Typical HFR sensors measure an AC resistance of the fuel cell based on a high-frequency ripple current. HFR is particularly sensitive to changes in RH. However, HFR is also highly sensitive to other fuel cell conditions, such as individual differences in overall membrane resistance, plate resistance, and contact resistance. Absolute HFR measurements are particularly susceptible to variation in contact resistance. Since the contact resistance of a fuel cell stack varies during operation with changes in compression force, due in part to swelling and contracting of membranes, absolute HFR measurements have heretofore not been employable in online hydration measurements of operating fuel cell stacks.
There is a continuing need for an online system and method for reliably measuring humidification of the fuel cell stack in operation. Desirably, the online system and method employs HFR measurements for monitoring and feedback control of fuel cell stack humidification.